Inkjet recording is a method of recording a letter or image (hereinafter sometimes simply referred to as an “image”) on the surface of a recording medium by ejecting an ink composition as small liquid droplets from fine nozzles. Examples of the inkjet recording system which is practically used include a method of converting an electrical signal into a mechanical signal using an electrostrictive element and intermittently ejecting an ink composition stored in the nozzle head part to record a letter or an image on the surface of a recording medium, and a method of rapidly heating an ink composition stored in the nozzle head part at a portion very close to the ejection part to generate bubbles and intermittently ejecting the ink composition using the volume expansion due to bubbles to record a letter or an image on the surface of a recording medium.
The ink composition for inkjet recording is generally obtained, in view of safety and printing property, by dissolving a dye of various types in water, an organic solvent or a mixed solution thereof, and various properties required of the ink composition are more strict as compared with the ink composition for writing tools such as fountain pen and ballpoint pen.
Particularly, in recent years, an inkjet printer is employed for the preparation of printed matters for advertisement and in forming a color image using a plurality of ink compositions, the requirements are more severe.
This is because, in the color image formed using a plurality of ink compositions, if even one color present has poor color hue, the color hue impairs the color balance of the image as a whole and a high-quality image cannot be obtained.
In performing printing using an ink composition (particularly when a metal phthalocyanine-base dye is used as a coloring material), a phenomenon that the portion of high-duty printing such as solid printing (full solid of 100% duty) looks reddish (hereinafter referred to as a “bronzing phenomenon”) sometimes occurs. This gives rise to a non-uniform color balance as the entire image and decreases the image quality and in this respect, improvements are being demanded.
Furthermore, in recent years, a gloss paper is employed in many cases as a recording medium having a feeling close to photographic touch, however, due to a bronzing phenomenon caused by a specific color, the gloss feeling on the printed matter surface is fluctuated to seriously impair the feeling of image. Therefore, also from the standpoint of maintaining the gloss feeling of the image as a whole, improvements are being keenly demanded.
The printed matter prepared using the above-described ink composition is disposed not only indoors of course but also outdoors sometimes and therefore, is exposed to various lights (including sunlight) and outside airs (e.g., ozone, nitrogen oxide, sulfur oxide). To cope with this, development of an ink composition excellent in the fastness to light and gas is being made but the bronzing phenomenon is outstanding particularly in the case of using a cyan ink composition improved in the gas fastness from the aspect of dye itself, and in this respect, improvements are being demanded.
In the case where a dye having poor solubility in water is used in the ink composition for inkjet recording, it is known that a so-called bronzing phenomenon occurs when printing is performed on a recording medium having strong acidity.
To solve this problem, generally, an alcohol amine is added as the pH maintaining agent or the penetration into paper is intensified. The bronzing phenomenon is considered to fundamentally occur due to crystallization of a dye resulting from drying of the ink composition on the paper surface and the above-described method is effective from the standpoint of elevating the solubility to prevent the crystallization or attaining penetration of the ink composition into paper before the dye is crystallized. However, in the former method, the pH readily elevates to the vicinity of 11 by the addition of alcohol amines and this may cause corrosion of nozzles of the printer or adversely affect the human body. In the latter method, if the penetration is excessively intensified, a so-called bleeding phenomenon and deterioration in other printing qualities are brought about. Therefore, these methods are not perfect.
For solving these problems, an ink composition where 2-oxo-oxazolidone or 1,3-bis(β-hydroxyethyl)urea is added has been proposed as an alternative (see, for example, Patent Publication 1 identified below).
Also, an ink composition where a basic amino acid is added with an attempt to solve these problems has been proposed (see, for example, Patent Publication 2 identified below).
However, it is regrettably found from the results of various investigations and studies by the present inventors that even if these techniques are employed, no effect is provided on the elimination or reduction of a bronzing phenomenon in the case of an ink composition using a cyan-type dye which ensures excellent fastness (mainly, fastness to light and gas) of the printed matter.
Patent Publication 1:
JP-A-6-25575 (the term “JP-A” as used herein means an “unexamined published Japanese patent application).
Patent Publication 2:
JP-A-7-228810